MECHANISMS OF PYROLYSIS. Jim Jones

Transcription

1 MECHANISMS OF PYROLYSIS Jim Jones

2 WHAT IS PYROLYSIS? the thermal decomposition of carbonaceous materials in the absence of oxygen

3 WHAT IS PYROLYSIS? the thermal decomposition of carbonaceous materials in the absence of oxygen + where the system pressure is not constrained by the vapour pressure of water This is called hydrothermal decomposition

4 WHAT IS PYROLYSIS? the thermal decomposition of carbonaceous materials in the absence of oxygen + where the system pressure is not constrained by the vapour pressure of water the first step in combustion This is called hydrocharring

5 WHAT IS PYROLYSIS? the thermal decomposition of carbonaceous materials in the absence of oxygen + where the system pressure is not constrained by the vapour pressure of water the first step in combustion This is called hydrocharring How?

6 WHAT IS PYROLYSIS? the thermal decomposition of carbonaceous materials in the absence of oxygen + where the system pressure is not constrained by the vapour pressure of water the first step in combustion Heat in This is called hydrocharring

7 WHAT IS PYROLYSIS? the thermal decomposition of carbonaceous materials in the absence of oxygen + where the system pressure is not constrained by the vapour pressure of water the first step in combustion Decomposition reactions This is called hydrocharring

8 WHAT IS PYROLYSIS? the thermal decomposition of carbonaceous materials in the absence of oxygen + where the system pressure is not constrained by the vapour pressure of water the first step in combustion Mass transfer out This is called hydrocharring

9 WHAT IS PYROLYSIS? the thermal decomposition of carbonaceous materials in the absence of oxygen + where the system pressure is not constrained by the vapour pressure of water the first step in combustion Mix with O 2 This is called hydrocharring

10 WHAT IS PYROLYSIS? the thermal decomposition of carbonaceous materials in the absence of oxygen + where the system pressure is not constrained by the vapour pressure of water the first step in combustion Combust This is called hydrocharring

11 WHEN THERE IS NO AIR Pyrolysis forms three products: Charcoal Condensable liquids (tar) Non condensable gases

12 WHAT IS THE ISSUE? A BLACK BOX

13 WHAT IS THE ISSUE? We set inputs We observe outputs Feedstock type Moisture content Particle size Additives Heating rate A BLACK BOX Yield Properties Soak temperature Residence time

14 WHAT IS THE ISSUE? We set inputs We observe outputs Feedstock type Moisture content Particle size Additives Heating rate To optimise pyrolysis and to dynamically control the reactions, we need to understand the mechanisms Yield Properties Soak temperature Residence time

15 WHAT IS THE ISSUE?

16 QUALITATIVELY...we understand the mechanisms at play. The description of the pyrolysis process is particularly challenging because it evolves a great deal of physical and chemical transformations and produces a large number of product species. As a result, existing models aiming to predict the rates or yields of the released pyrolytic volatiles are still supported by empirical data... [Figure and quote from Neveet al., 2011]

17 Primary Rxns 1 2 Biomass KINETIC MODEL 1 Secondary Rxns gas 4 volatiles Endothermic 3 char Exothermic Each reaction has: 1. a heat of reaction 2. a rate Mechanism proposed by Shafizadeh, 1975

18 Primary Rxns Biomass Secondary Rxns gas volatiles char Each reaction has: 1. a heat of reaction 2. a rate 4 5 KINETIC MODEL 1 1. Heat of Reaction Endothermic 4 5 Exothermic 2. Rate of Reaction Legend A E R k i = Ae i E i Reaction No. H [kj/kg] / RT =pre-exponential factor [1/s] =activation energy [kj/mol] = gas constant = [J/mol/K] Mechanism proposed by Shafizadeh, 1975 [Data cited in Fantozzi, 2007]

19 Primary Rxns Biomass Secondary Rxns gas volatiles char Each reaction has: 1. a heat of reaction 2. a rate 4 5 KINETIC MODEL 1 1. Heat of Reaction Endothermic 4 5 Exothermic 2. Rate of Reaction Legend A E R k i = Ae i E i Reaction No. H [kj/kg] / RT =pre-exponential factor [1/s] =activation energy [kj/mol] = gas constant = [J/mol/K] Reaction No. A E ref Temp [K] E E E E E Mechanism proposed by Shafizadeh, 1975 [Data cited in Fantozzi, 2007]

20 Biomass dm dt dm Primary Rxns B gas,1 dt dm dt dm dt dm dt dm char,1 gas,2 char,2 dt = m volatiles B = m ( k B = m = m = m k = m B 1 B k 1 k 3 2 volatiles k volatiles k 4 k 2 m 5 + k Secondary Rxns gas 3 ) volatiles volatiles char ( k k KINETIC MODEL 1 5 ) 1. Heat of Reaction Endothermic 4 5 Exothermic 2. Rate of Reaction Legend A E R k i = Ae i E i Reaction No. H [kj/kg] / RT =pre-exponential factor [1/s] =activation energy [kj/mol] = gas constant = [J/mol/K] Reaction No. A E ref Temp [K] E E E E E [Data cited in Fantozzi, 2007]

21 KINETIC MODEL 2 Biomass 1 2 (volatiles + gases) (volatiles + gases) 2 +char 2 char 1 Reaction order is now flexible cfkinetic MODEL Biomass 3 gas 4 volatiles 5 char [Mechanism proposed by Koufopanos et al., Elaborated by Babu and Chaurasia, 2002]

22 The components of biomass each have different decomposition kinetics KINETIC MODEL 3 tar 4 gas Cellulose Hemicellulose Lignin 1 2 intermediate 3 char + gas cfkinetic MODEL Biomass gas 4 volatiles cfkinetic MODEL 2 1 Biomass (volatiles + gases) (volatiles + gases) 2 +char char 2 char 1 Mechanism proposed by Miller and Bellan, 1997

23 The components of biomass each have different decomposition kinetics KINETIC MODEL 3 tar 4 gas Cellulose Hemicellulose Lignin 1 2 intermediate 3 char + gas cfkinetic MODEL Biomass gas 4 volatiles cfkinetic MODEL 2 1 Biomass (volatiles + gases) (volatiles + gases) 2 +char char 2 char 1 Mechanism proposed by Miller and Bellan, 1997

24 NETWORK MODELS...are focussed on emissions They are... the most fundamental pyrolysis models... which aim to describe the physical and chemical phenomena taking place during a particle s devolatilization. De Jonget al use: FG-DVC - Function Group - devolatilization, vaporisation and crosslinking model It uses TG-FTIR data. [Quote from de Jonget al., 2007]

25 NETWORK MODELS How they work Network models are based on a structural description of the parent fuel...during devolatilization, the macromolecular fuel structure is changed as a result of depolymerization, vaporization and cross-linking of the fuel matrix. This causes the breakup of existing bridges connecting the aromatic rings and the formation of new bridges at rates described by network statistics. As a result, gaseous products and tars are formed, while the solid particle converts into carbonaceous char. FG-DVC models have two parts: A functional group model (FG)-describes the gas evolution as well as the elemental and functional group compositions. Depolymerization, vaporization and cross-linking model (DVC) - determines the amount and molecular weight of macromolecular fragments. The lightest of these evolve as tar. [Quote from de Jonget al., 2007]

26 OUTPUT = f{inputs} MODEL Neve et al. (2011) suggest that empirical relationships can be developed to approximate the pyrolysis behavior of most biomasses. Moreover, the literature data structured by the authors is related to a wide range of operating conditions. We set inputs We observe outputs Feedstock type Moisture content Particle size Additives Heating rate A BLACK BOX Yield Properties Soak temperature Residence time [Model proposed by Neve et al., 2011]

27 OUTPUT = f{inputs} MODEL Dry ash free char Gas Liquids Water [Model proposed by Neve et al., 2011]

28 DON T FORGET HEAT AND MASS TRANSFER Larger particles have heat and mass transfer limitations It is important to hold larger particles for longer time so that temperatures equilibrate Biot number = Heat transfer resistance Mass transfer resistance

29 SUMMARY There are many approaches to understanding pyrolysis mechanisms. Five are presented here, from purely empirical, to kinetic models with empirical constants to fundamental mechanistic models. The degree of complexity has to relate to the end-use of the data.